HVDC Applications

Overview

Applications

Our power supply system is changing fundamentally. Dwindling resources, environmental considerations and technological progress will lead to a fundamental change of this landscape in the coming decades. High Voltage Direct Current (HVDC) systems are an ideal supplement to the existing AC power grid and are going to play a key-role in the new energy era by:

economical and most efficient transmission of electrical power over long distance,

connecting asynchronous grids or grids with different frequencies,

providing controlled power supply in either direction,

offering grid access for onshore and offshore power generation from renewable energy sources,

performance improvement and acting as a “firewall” against blackouts in existing overloaded AC grids.

In an AC network the converter station as part of the HVDC system converts the electric power into DC. In an additional converter the electrical power is converted back to AC. The converters can be located in one place as a back-to-back HVDC system, or electrical power can be transmitted from one converter station to another over long distance via an overhead line or cable. In both cases HVDC increases the transmission capacity and system stability very efficiently.

Siemens offers as a pioneer in HVDC technology the whole portfolio for all kind of HVDC applications:

Bulk Power UHV DC (Ultra High Voltage Direct Current) up to 10 GW for 800 kV

Siemens provides efficient and sustainable solution for all applications with DC transmission.

Upgrades and Refurbishments

Inter-Island Connector, New Zealand

DC systems have been available for decades. Once built, restrictions may change or the power demand may increase, making an upgrade of the HVDC system necessary. The latest DC technology offers more efficient and space-saving solutions compared to previously used technologies (History; Innovations), meaning that refurbishment of the existing HVDC system is called for.

With the New Zealand Inter-Island Connector Pole 2 and Pole 3 Siemens has set a milestone for the upgrade of an existing HVDC link during operation.

Interconnections

Siemens offers different solutions for interconnections of asynchronous grids. This HVDC applications offer bidirectional control of power flow which can be used for power trading, e.g. Britned, an HVDC link from UK to Netherland.

DC back-to-back is frequently used to couple asynchronous neighboring AC networks (Black Sea project, Georgia), e. g. two or more national grids, with different frequency or voltage control characteristics. A back-to-back connection with HVDC overcome these difficulties by converting the electrical parameters from one AC system into DC and back from DC to the electrical parameters from the other AC system. HVDC enables a controllable power flow.

Furthermore, interconnection with a back-to-back HVDC link will not increase the short-circuit power of the systems. In addition, it prevents the spread of cascading disturbances (a firewall for blackout prevention).

Island – Interconnections

When an island has to be connected to the main grid or asynchronous transmission systems separated by the sea have to be coupled, subsea cable HVDC is the solution. Siemens has already implemented several of these HVDC applications. For example, the Storebælt HVDC transmission system connects the grid in Jutland/Funen, as part of the UCTE system with the Zealand Grid, which is part of the NORDEL system, via subsea cable.

The Cometa project incorporates an HVDC link with one converter station on the Spanish mainland in St. Ponsa near Palma de Mallorca and another converter station on the island of Majorca as an island interconnection that reduces emissions of climate-damaging CO2 by 1.2 megaton. This amount of gas would otherwise be produced if this system was not in use, for example by the construction of a new petroleum-fired 400 MW power plant on Majorca.

Long-Distance Transmission via Overhead Line

DC transmission via overhead lines

The most economical solution for DC transmission is interconnection via overhead lines (OHL).

DC overhead lines are HVDC applications that use more narrow transmission corridors for the same power transfer than compared to their AC counterparts and this is often a crucial issue for opting for DC transmission. In the future DC may also be used to increase the power transfer volume through existing rights-of-way by converting AC lines to DC.

All OHL sections are subject to a higher risk of fault (from lightning strikes etc.) than insulated cables and therefore HVDC has the capability to cope with faults on the DC line. Here, the full-bridge configuration is used.

Long-Distance Transmission via Cable

Basslink, a long-distance cable link connecting the states of Tasmania and Victoria in Australia

Gas-Insulated Transmission Lines (GIL), an innovative Siemens underground system for AC transmission can be an economically viable alternative to conventional power cables connecting back-to-back HVDC, especially for providing bulk power over short distances.

Bulk Power Transmission

UHV DC Yunnan-Guangdong, China

As the demand for electric power steadily rises, many existing AC transmission systems have already reached their capacity limits. At the same time, the share of renewable energy in the energy mix is growing. However, wind, solar, and hydropower is mostly generated far away from load centers.

This is why there will be a need for bulk power transmission corridors that can efficiently handle long-distance power transmission in the gigawatt range.

Siemens ultra high voltage direct current (UHV DC) technology is an HVDC application for the transmission of large power amounts and takes DC power transmission to the next level:

Ability to efficiently transmit bulk power over very long distances: up to 10 GW at a voltage of ±800 kV

UHV DC offers a key to increased performance and robustness of the transmission grid to keep pace with the steadily growing energy demand.

The world’s first ultra-high-voltage direct current (UHV DC) transmission system connects the Yunnan and Guangdong provinces in China (Yunnan-Guangdong project, China).

Thanks to thorough R&D efforts, Siemens portfolio covers the entire range of components required for 800-kV DC power transmission and supplies complete UHV DC systems from a single source.

Multi-terminal Applications and Grid Enforcement

Grid enforcement by HVDC Hudson, USA

Multi-terminal applications

The concept of HVDC multi-terminal systems has always been a consideration from the very beginning of HVDC development. However, only a few systems have been constructed in line-commutated converter technology worldwide to date.

New HVDC applications with VSC technology provide significant benefits for multi-terminal or even DC grid systems. Excellent performance is provided with HVDC PLUS in full-bridge topology allowing selective fault clearing on overhead lines in radial multi-terminal systems. The latest development from Siemens, the DC Compact Switchgear including a fast disconnector switch, rounds out the range of technology available for multi-terminal configurations.

Grid enforcement

HVDC helps to prevent bottlenecks and overloads in power grids by means of systematic power flow control. The function of HVDC which is decisive for power security is that of an automatic firewall. This firewall function prevents the propagation of a disturbance in the system at all times, similar to traffic lights on the “power highway“. As soon as the disturbance has been cleared, power transmission can immediately be restored.

A further HVDC application which is highly important for grid enhancement is its integration into the complex interconnected AC system. The driving factors behind these hybrid solutions are basically lower transmission costs as well as the possibility of bypassing heavily loaded AC systems. (HVDC Neptune project, US, HVDC Hudson project, US)

Grid Access for Offshore Applications

HVDC offshore platform BorWin2, Germany

The grid of the future must be secure, cost-efficient and environmentally compatible at the same time. The combination of these three tasks can only be achieved if ideas, intelligent solutions and innovative technologies come together. Wind power is namely comparatively volatile and subject to severe fluctuations and, when used efficiently, is mostly located offshore. These fluctuations have a significant impact on voltage and frequency stability in the power grid. Therefore, “Green Energy” is an issue for Grid Code compliance and power quality in transmission and distribution systems.

Siemens offers perfect solutions for HVDC offshoreapplications. HVDC PLUS is the ideal space-saving solution to provide grid access to offshore wind farms with its dynamic fast control. This helps to make regenerative energy sources fit for the grid, and to connect them according to the conditions of each particular Grid Code by providing the required voltage quality at the grid coupling point.

The HVDC PLUS solution, based on voltage-sourced converter technology with its bi-directional power transmission, is the access technology of choice whenever black-start capability is deemed necessary.

DC for Megacities

Trans Bay Cable, USA

Today cities mainly rely on AC grids. Using DC transmission for a city’s power in feed decreases overall transmission grid congestion and increases overall security and reliability of the electrical system in the area. It helps to meet the increasing power demand in an energy-efficient and cost-effective manner for the future. Furthermore, it also reduces the need to build additional new power plants in or near the city.

The Hudson project in New York, USA, for example, offers the opportunity to add renewable sources in the future.

Crucial for converter sites in urban areas, the HVDC PLUS solution utilizes a minimum amount of space and minimizes environmental impact such as visual impact, audible noise and transport during construction.